Re: [PATCH 2/2] stop_machine: Apply smp_store_release() to multi_stop_data::state

[Rong Tao]

On 10/24/23 7:01 PM, Mark Rutland wrote:
> On Fri, Oct 20, 2023 at 10:43:34PM +0800, Rong Tao wrote:
> > From: Rong Tao <rongtao@xxxxxxxx>
> >
> > Replace smp_wmb()+WRITE_ONCE() with smp_store_release() and add comment.
> >
> > Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
> > ---
> >   kernel/stop_machine.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > index 268c2e581698..cdf4a3fe0348 100644
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -183,8 +183,10 @@ static void set_state(struct multi_stop_data *msdata,
> >   {
> >   	/* Reset ack counter. */
> >   	atomic_set(&msdata->thread_ack, msdata->num_threads);
> > -	smp_wmb();
> > -	WRITE_ONCE(msdata->state, newstate);
> > +	/* This smp_store_release() pair with READ_ONCE() in multi_cpu_stop().
> > +	 * Avoid potential access multi_stop_data::state race behaviour.
> > +	 */
> > +	smp_store_release(&msdata->state, newstate);
> This doesn't match coding style:
>
> 	/*
> 	 * Block comments should look like this, with a leading '/*' line
> 	 * before the text and a traling '*/' line afterwards.
> 	 */
>
> See https://www.kernel.org/doc/html/v4.10/process/coding-style.html#commenting
Thanks, Mark, I'll fix the comment in next patch version.
> I don't think the "Avoid potential access multi_stop_data::state race
> behaviour." text is all that helpful, and I think we can drop that.
>
> In general, it's unusual to pair a smp_store_release() with READ_ONCE(), and
> for that to work it relies on dependency ordering and/or hazarding on the
> reader side (e.g. the atomic_dec_and_test() is ordered after the READ_ONCE()
> since it's an RMW and there's a control dependency, but a plain read could be
> reordered w.r.t. the READ_ONCE()). So we probably need to explain that if we're
> going to comment on that smp_store_release().
>
> Peter, might it be worth replacing the READ_ONCE() with smp_load_acquire() at
> the same time? I know it's not strictly necessary given the ordering we have
> today, but it would at least be obvious.

After I wait for Peter to reply to this message, I will write a patch 
based on Peter's suggestion.

Rong Tao.

> Mark.
>
> >   }
> >   
> >   /* Last one to ack a state moves to the next state. */
> > --
> > 2.41.0